Spectroscopic or spectrometric analysis is a broad field in which the composition and properties of an analyte in any phase, viz, gas, liquid, solid, are determined from the residual electromagnetic spectra arising from the interaction (eg. absorption, luminescence, or emission) of the analyte and energy. One aspect of chemical analysis, known as absorptiometry, involves interaction of radiant energy with the analyte of interest. One such method is known as absorption photometry, in which the optical absorption of samples are measured. The absorption is the amount of energy absorbed by the sample. In a simple spectrophotometer the studied sample material is placed in a container, also known as a cuvette or sample cell. Electromagnetic radiation (light) of a known wavelength, λ, (i.e. ultraviolet, infrared, visible, etc.) and intensity I0 is incident on one side of the cuvette. A detector, which measures the intensity of the transmitted light, I is placed on the opposite side of the cuvette. The length that the light propagates through the sample is the distance d. Most standard UV/visible spectrophotometers utilize standard cuvettes which have up to 1 cm path lengths, and often much shorter, and normally hold 50 to 2000 μL of liquid sample. For a sample consisting of a single homogeneous substance with a concentration c, the light transmitted through the sample will follow a relationship know as the Beer-Lambert Law: A=εcd where A is the absorbance (also known as the optical density (OD) of the sample at wavelength λ, where OD=the −log of the ratio of transmitted light to the incident light), ε, is the absorptivity or extinction coefficient (normally at constant at a given wavelength), c is the concentration of the sample and d is the path length of radiation through the sample. In most spectrophotometers the path length, d, is fixed.
It is known that usually the ε is high resulting in that cuvettes with small d must be used in order to record any transmission. It is also known that it is possible to alter the path length, d, for measurement of highly concentrated samples. This is used to provide a possibility of choosing an appropriate path length for different measurements, and for the measurement, a single path length and single wavelength measurements are used. Such systems are e.g. known from WO 2007/126389, U.S. Pat. No. 6,249,345 and DE 85 33 381.
It is also known that it is possible to use a variable path length during one measurement, and to use a regression line analysis of the resulting path-length dependent attenuations to determine the concentration of a sample. This is shown in U.S. Pat. No. 7,808,641, and is referred to as slope spectroscopy. However, slope spectroscopy requires a moveable probe which is inserted into the sample material, and which is adjusted to different pathlengths. The complex set-up and interaction with the sample material to be measured makes this method cumbersome and expensive, and also provides limited practical use since the described method can only be used for certain types of liquid solutions. Further, it is also difficult to obtain adequate calibration.
There is therefore a need for a faster and simpler method and apparatus for estimating the concentration of an analyte in a sample material, and in particular a solid material, such as in wood, which alleviates the above-discussed drawbacks of the prior art.